Marinate the fish pieces with 1 tsp turmeric and salt to taste and keep aside for 10 minutes.
Meanwhile heat the mustard oil to smoking point and lower the flame, fry the fish pieces in it till golden brown on both sides, drain and keep aside.
In the same oil fry the vertically cut potatoes till brown on both sides, drain and keep aside.
Add more mustard oil if needed or else in the same oil add whole cumin and sugar, let it splutter and add the chopped tomatoes, the slit green chillies and 1/2 tsp turmeric.
Cook the tomatoes till mushy and then add the hot water fried potatoes and salt to taste, let it boil for 5 minutes on a low flame.
Add the cumin and coriander powder, simmer for 1-2 minutes and then add the fried fish pieces and let it simmer for 4-5 minutes on a low flame.
Switch off the flame and garnish with chopped coriander.